1. Field of the Invention
This invention generally relates to current source devices and more particularly to a current source device adapted to supply a predetermined amount of current to a load irrespective of the magnitude of the load.
In the event that a supply voltage to a current source device fluctuates when supplying a constant current from the device to a load, it has been desirable to change the constant current at the same rate of change as that of the supply voltage. Such an expedient will be described by referring, by way of example, to a semiconductor pressure transducer used for measurement of pressure of a mixture (gasoline plus air) supplied to a car engine. The semiconductor pressure transducer is known, in which a thin diaphragm is formed at the center of a silicon single crystal plate, gauging resistors are formed on the surface of the diaphragm by impurity diffusion layers, and the gauging resistors are connected to form a sensor of a bridge circuit. The semiconductor pressure transducer is usually connected to a constant current source device and driven by a constant current. Accordingly, the output voltage of the semiconductor pressure transducer is proportional to the current supplied to the bridge circuit. The output voltage of the semiconductor pressure transducer is amplified at an amplifier, and the amplified output signal is digitized at an A/D converter. In this manner, an analog quantity representative of a pressure of the mixture produced from the semiconductor pressure transducer is converted into a digital value. The constant current source device, amplifier and A/D converter are all driven by a battery carried in a car or by a DC voltage which is converted from an output voltage of the battery by means of a DC-to-DC converter. The driving voltage, however, fluctuates, depending on such factors as the charged state of the battery and the magnitude of load on the battery. Generally, the A/D converter performs A/D conversion referenced to a supply voltage fed to the A/D converter. Accordingly, a decrease in the supply voltage, for example, leads to a decrease in the reference voltage for the A/D converter, with the result that the output of the A/D converter increases beyond a correct value, even if the voltage of the input signal to the A/D converter remains unchanged. In order to obtain a correct output, therefore, it is required that the amount of current fed from the constant current supply device to the pressure transducer be reduced at the same rate as that of the decrease of the driving voltage.
2. Description of the Prior Art
Various types of current supply circuits in the form of integrated circuits have hitherto been available. A typical example of a prior art current supply circuit is illustrated in a circuit diagram of FIG. 1, which may be referred to in "Analysis and Design of Analog Integrated Circuits" by Paul R. Grey and Robert G. Meyer, published by John Wiley & Sons (1977), pp. 200, 201, 206, 207, 236 and 273, for example.
As shown, a resistor 1 has one end connected to a supply voltage Vcc and the other end connected to the collector of a transistor 11. The transistor 11 has an emitter connected to a common power supply line via a resistor 3 and a base short-circuited to its collector. A transistor 12 has a base connected to the base of the transistor 11, an emitter connected to the common power supply line via a resistor 2 and a collector connected to a terminal 22. A load (not shown) may be connected between a terminal 21 connected to the supply voltage Vcc and the terminal 22, and an output current Ic serving as a load current is fed to the load.
The operation of this circuit will now be described. If the transistors 11 and 12 have such large current-amplification factors .beta..sub.11 and .beta..sub.12 that the base current can be neglected (this assumption is valid for primary approximation since NPN transistors generally have a current-amplification factor .beta. of 100 or more), the output current Ic can be expressed as, ##EQU1## where V.sub.T : V.sub.T =kT/q (K, T and q will be described later)
I.sub.s11 : saturation current of transistor 11 PA1 I.sub.s12 : saturation current of transistor 12 PA1 R.sub.2 : resistance of resistor 2 PA1 R.sub.3 : resistance of resistor 3
The second term in brackets "[ ]" represents a difference voltage between the base/emitter voltages of the transistors 11 and 12 and this difference voltage amounts to 150 mV, at the most, for a current ratio of about 100. Since, in general applications, Iref.R.sub.3 is set to be sufficiently larger than the value of the difference voltage, the output current Ic can be approximated by the following equation: ##EQU2##
Considering operations characteristic of the FIG. 1 circuit, it should be understood that the transistors 11 and 12 have an equal emitter voltage, and that Ic changes in proportion to changes of Iref.
In connection with the current source circuit shown in FIG. 1, so-called ratio metricity will now be discussed which characterizes a relationship in which the output current changes at the same rate of change as that of the supply voltage Vcc. Denoting the base/emitter voltage of the transistor 11 by V.sub.BE11, the current Iref is written as, ##EQU3## whereas R.sub.1 is a resistance of the resistor 1. Accordingly, a rate of change of Iref, designated by .gamma., is related to a rate of change of Vcc, designated by .xi., as follows: ##EQU4##
Therefore, ##EQU5##
Since, in equation (4), Vcc&gt;Vcc-V.sub.BE11 stands, the rate of change .gamma. of Iref is always larger than the rate of change .xi. of Vcc. As a result, there is no ratio metricity between Vcc and Iref. The ratio metricity between Vcc and Iref is defined so that the change rate .gamma. of Iref equals the change rate .xi. of Vcc. Considering that the transistors 11 and 12 in the FIG. 1 circuit are connected in a so-called current mirror fashion and the output current Ic is in proportion to Iref, as will be seen from equation (2), ratio metricity is also excluded between the supply voltage Vcc and the output current Ic.